Laser communication technology can improve the performance of space communication systems by offering higher carrier frequency and information bandwidth. The small beam divergence resulting from the short operating wavelength can also lead to an improved channel security and, more importantly, can permit communication systems to use a smaller aperture antenna while providing increased channel throughput compared to radio frequency systems. The resulting reduction in size and mass of the communication system can lead to an increased payload capacity for the host spacecraft.
For planetary missions, the reduction in communications system size can also lead to a simplified spacecraft design. The large RF antenna currently used by communication systems can restrict the field-of-view of scientific instruments. It also imposes constraints on the attitude control of the spacecraft because the antenna must be kept pointed at the receiving site. In contrast, a smaller optical communications instrument can be articulated independent of the spacecraft attitude, and can permit more options for spacecraft control. A smaller communications package also eliminates the need for an unfurlable antenna and a large scan platform boom, thereby simplifying the spacecraft design. In some cases, the reduction in size can also permit a wider diversity of launch vehicle options. Smaller spacecraft currently being proposed for the planetary and space physics missions, such as the Explorer and Discovery-class spacecraft, will impose stringent demands on the communication system. For these missions, laser communication technology offers an attractive method of providing increased data throughput while at the same time decreasing the mass and size of the communications subsystem. Additionally, lasercom technology can be applied to near-Earth space communication systems. The high information bandwidth of the optical channel can permit intersatellite crosslinks to operate at data rates in excess of several hundred megabits per second while at the same time offering improved channel security and decreasing the dependency on foreign ground tracking stations.
The narrow transmit beamwidth of the lasercom system, on the other hand, can impose stringent demands on the pointing control accuracy of the instrument. Inaccurate beam pointing can result in large signal fades at the receiving site and a severely degraded system performance. Since the uncertainty in the spacecraft attitude is much larger than the beamwidth, an initial acquisition process needs to be performed to acquire the receiver location. Furthermore, since the spacecraft attitude errors due to deadband cycle and random platform jitter are also much larger than the transmit beamwidth, a dedicated pointing control subsystem is required to reduce the signal loss due to pointing error. Such a subsystem must be capable of tracking the receiving station such that the residual pointing error is less than approximately 20% of the diffraction-limited beamwidth.
The required pointing acquisition and tracking subsystems for laser communication instruments have been developed and tested for several systems in various stages of flight readiness. However, these previous subsystem designs tend to be very complex as the designs generally employ decade-old technology. For example, two separate detectors are required for spatial acquisition and tracking, and two beam steering mechanisms are required for line-of-sight stabilization and point-ahead compensation. Because of the design complexity, extensive efforts were required to ensure functionality and to achieve the desired reliability. As a result, these systems tend to be very costly and, in some cases, more massive than comparable RF technologies.
The conventional design approach is to sense the beacon line-of-sight jitter using a high speed tracking detector and to control said jitter using a high-bandwidth steering mirror. This design approach does indeed stabilize the beacon line-of-sight, but unfortunately requires a separate beam steering mirror to provide the point-ahead angle in order to compensate for the relative motion between transmit and receive systems. Furthermore, a wide field-of-view acquisition detector is generally required to permit initial signal acquisition. The complexity of this conventional design approach has led to higher development costs for lasercom systems.